I had a single user receiving this error while trying to run native CRM reports as well as any custom report that used the Advanced Find table alias CRMAF.....it turned out that they had changed 1 letter in their active directory logon name and CRM still had the old version. Since this field is locked down in CRM and can't be changed I went into their AD user account and changed it back to the orginal version that matched what was in CRM. I had the user log-off and log back onto their machine with the restored user logon name and the reports ran fine without error.
There is also a Microsoft KB article that deals with this issue although it doesn't direct the reader to compare the CRM User Logon Name with the AD User Logon Name. That article can be found at http://support.microsoft.com/kb/927021 | 
